The name of the ionic compound Cr3(PO4)2 is
chromium(III) phosphate
trichromium diphosphate
chromium phosphide
chromium(II) phosphate
The correct answer and explanation is :
The correct name for the ionic compound Cr₃(PO₄)₂ is chromium(II) phosphate.
Explanation:
To properly name an ionic compound, we must follow the standard rules for naming binary ionic compounds, which typically consist of a metal and a nonmetal, as well as compounds involving polyatomic ions. Here’s the step-by-step breakdown for naming Cr₃(PO₄)₂:
- Identifying the Metal Ion: The metal in the compound is chromium (Cr). Chromium is a transition metal, and like other transition metals, it can form ions with multiple charges. In this case, we need to determine the charge of the chromium ion.
- The phosphate ion (PO₄³⁻) has a charge of -3.
- The formula Cr₃(PO₄)₂ suggests there are 3 chromium ions and 2 phosphate ions. To balance the charges, the total positive charge from the chromium ions must equal the total negative charge from the phosphate ions.
- Since the phosphate ions contribute a total of -6 charge (2 × -3 = -6), the total positive charge must be +6 to neutralize the compound.
- Therefore, each chromium ion must have a charge of +2 (because 3 × +2 = +6). Hence, the chromium ion in this compound has a charge of +2.
- Naming the Metal Ion: In the case of transition metals that can have more than one possible charge, we specify the charge in parentheses after the metal’s name. Since chromium has a charge of +2 in this compound, we use chromium(II).
- Identifying the Polyatomic Ion: The polyatomic ion in the compound is phosphate (PO₄³⁻). The name of this ion is always phosphate.
- Combining the Names: Putting it all together, the compound is named chromium(II) phosphate, where “chromium(II)” indicates the +2 charge on the chromium ions, and “phosphate” is the name of the polyatomic ion.
Why Other Options Are Incorrect:
- Chromium(III) phosphate: This would imply that chromium has a +3 charge, which is not the case here. The correct charge is +2.
- Trichromium diphosphate: This is an incorrect naming convention. It suggests there are three chromium atoms and two phosphate groups, but it omits the necessary indication of chromium’s charge.
- Chromium phosphide: Phosphide refers to the ion PO₄³⁻ in a compound with a metal, but it does not apply here, as “phosphide” is typically used for ionic compounds where the metal forms a bond with phosphorus (P), not phosphate (PO₄³⁻).